Howdy,

Some vacuum retard trivia....

The vacuum retard system works only at idle. The vacuum port that applies vacuum to the retard canister is on the throttle body right at the throttle. As soon as the throttle tips in, the port is uncovered and the vacuum retard is cancelled. The retard amount is listed in the M117 workshop manual as 15 degrees when active.

The vacuum signal from the throttle body "should" go to a two way valve (vacuum relay unit) mounted on the firewall.

The two way valve is activated by two of the three relays mounted on the passenger side of the firewall. The purpose of this circuit is to activate the two way valve (cancel vacuum retard at idle) when the 100 deg C switch is active.

The 100 deg C switch is on the front of the engine where the water exits the intake manifold and goes into (out of?) the water pump. It is brass in color with a red center and has one terminal sticking up. There is a single terminal that goes to it. This wire is brown/yellow (mine looks solid brown) but should be covered by a sheath and is therefore hard to see. The switch will have "100 C" stamped on the side if you can see it. You will be able to disconnect this terminal and manually ground it to the engine to test the aux fan and vacuum retard. My fan works, my vacuum retard doesn't. I've got the vacuum servo hooked directly to the vacuum sense port on the throttle body and not to the 2 way switch.

There is an A/C switch on the output of the A/C dryer(?) in front of the condenser. The sensor is clamped to the freon line and has two wires going to it. The brown/yellow wire goes to the aux fan relay where it activates the aux fan. The solid brown line goes to a ground point underneath the battery I think. This one should not activate the 2 way valve so there shouldn't be any affect on idle vacuum retard.

I've heard that some systems remove vacuum retard when the A/C compressor is running. That would make sense (bump the idle speed), but I don't see it in this diagram. I'm pretty sure the only thing that is supposed to cancel vacuum retard is the 100 deg C switch.

Sholin

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What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.